1. Field of the Invention
The present invention relates to an optical cable which can suitably be installed on land, as well as to a method of installing the same. This invention also is concerned having an optical transmission line comprising such optical cable.
2. Description of the Background Art
Hybrid optical transmission lines have been known, which employ a first optical fiber having a positive chromatic dispersion at signal light wavelength, and a second optical fiber connected to the first optical fiber having a negative chromatic dispersion at the same wavelength. In regard to such hybrid optical transmission lines, reference is made to, for example, Document 1, M. Tsukitani et al., xe2x80x9cLow-loss dispersion-flattened hybrid transmission lines consisting of low-nonlinearity pure silica core fibers and dispersion compensating fibersxe2x80x9d, Electron. Lett., Vol. 36, No. 1, pp. 64-66 (2000).
The optical transmission lines disclosed in this document can effectively suppress degradation of the signal attributable to a non-linear optical effect, by using the first optical fiber having an enlarged effective area for the upstream part of the optical transmission line where the signal light power is strong. In addition, by reducing the absolute value of overall chromatic dispersion, degradation of the signal can be suppressed. The hybrid transmission line disclosed in this document can be suitably used for optical transmission systems that transmit a large volume of information over a long distance.
Dispersion managed optical transmission line has been known in which the above-mentioned first and second optical fibers are connected alternately. In this dispersion managed optical transmission line, the absolute value of chromatic dispersion at each point can be increased so as to suppress degradation of the signal caused by four-wave mixing which is a kind of non-linear optical effect. The dispersion managed optical transmission line also can reduce the absolute value of overall chromatic dispersion to suppress degradation of the signal, and can be suitably used in optical transmission systems which transmit a large volume of information over a long distance.
In the meantime, the hybrid optical transmission line comprising optical cables has been also proposed. By way of example, reference is made to Document 2, M. Morimoto, et al., xe2x80x9cStudy on mechanical and optical characteristics of reverse dispersion fiber cablesxe2x80x9d, International Wire and Cable Symposium Proceedings 1999, pp. 51-54, or to Document 3, L. Gruner-Nielsen, et al., xe2x80x9cCabling of dispersion compensating fibersxe2x80x9d, International Wire and Cable Symposium Proceedings 1999, pp. 483-487.
The length of optical cable that can be installed on land is generally not exceeding about 10 km. Thus, a feasible method is to construct a hybrid optical transmission line or a dispersion managed optical transmission line by manufacturing and installing, on land, the first optical cable containing only a plurality of first optical fibers and the second optical cable containing only a plurality of second optical fibers, and then the optical fibers contained in the first and second optical cables, respectively, are connected together by fusion splicing.
In general, the effective area of the first optical fiber having a positive chromatic dispersion at the signal light wavelength is larger than that of the second optical fiber having a negative chromatic dispersion at the same wavelength. Connecting these two types of optical fibers having different effective areas simply by fusion splicing produces a big splicing loss. A method has been proposed, therefore, to reduce the splicing loss by heating the spliced joints to cause diffusion of dopants, after the fusion splicing operation. Such a method is disclosed, for example, in Japanese Patent Application Laid-Open No. H3-130705.
It is an object of the present invention to provide an optical cable having a reduced splicing loss while affording high efficiency of installation work, and so is suitable to be installed on land. This invention also provides a method of installing optical cables on land to construct an optical transmission line, as well as the optical transmission line itself comprising such optical cables.
In order to achieve this object, such an optical cable is provided, that a first optical fiber having a positive chromatic dispersion at 1.55 xcexcm wavelength and a second optical fiber having a negative chromatic dispersion at the same wavelength having been connected with each other by fusion splicing to form a joint are accommodated therein.
The optical cable may have a length which is not less than 1 km but not more than 10 km. The ratio of the effective area of the first optical fiber to that of the second optical fiber may be 0.5 or less, or 2.0 or more. The distance between the spliced joint and the adjacent end of the optical cable may be not less than 100 m. The absolute chromatic dispersion value of each of the first and second optical fibers may not be less than 10 ps/nm/km. The joint part may have been subjected to heat treatment and re-coated after the fusion splicing operation. The splicing loss at the spliced joint may be not more than 0.3 dB. Also each of the first and the second optical fibers may be provided with information for identifying each type of optical fiber. Throughout this specification, values of characteristics are those to be obtained at the wavelength of 1.55 xcexcm, unless otherwise specified.
A method of installing an optical cable is also provided in which a first optical cable relating to the present invention and a second optical cable accommodating the same type of optical fibers as those in the first optical cable are installed on land, and then the optical fibers of the same type accommodated in the first and second optical cables are connected together by fusion splicing. Alternatively, a first and a second optical cables each relating to the present invention are installed on land, and the optical fibers of the same type accommodated in the first and second optical cables are connected together by fusion splicing.
The above and further objects and novel features of the invention will be more fully clarified in the following detailed description when the same is read in conjunction with the accompanying drawings. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.